The prothrombin time (PT) test was introduced to control oral anticoagulant therapy (OAT), which is used to treat patients with thrombotic disorders such as thrombophilia. This therapy is based on vitamin K antagonists such as warfarin (or coumarin) inhibiting the synthesis of coagulation factors (F II, F VII, F IX and F X) in the liver. Currently, the need for OAT is increasing worldwide. For example, in Finland warfarin medication was prescribed to 1.7% of the population in 2003 and the number of patients is growing due to the ageing population. Since OAT medication needs continuous control to prevent serious consequences of thrombosis or bleeding, it is not surprising that 800 million PT tests are performed each year throughout the world. The most commonly used PT tests are based on Quick's one-stage PT and Owren's method. The latter is most widely used in the Nordic and Benelux countries as well as in Japan, whereas the Quick PT is the approach used elsewhere, accounting for about 95% of all PT tests performed.
When the International Normalised Ratio (INR) and International Sensitivity Index (ISI) were introduced by the World Health Organization (WHO), the aim was to harmonise PT results for oral anticoagulant therapy (5): PT results for a certain plasma or whole blood sample should be the same in INR units regardless of the reagent, instrument or method used. Today each commercial coagulation reagent (i.e. thromboplastin) for PT is calibrated against the primary WHO reference preparation. The results are used to calculate the relative sensitivity of the thromboplastin reagents declared in ISI (International Sensitivity Index). The INR results are then calculated according to the formula: INR=[PT ratio]ISI. However, increasing use of the INR format has led to appreciation of its limitations and recently it was found, that the agreement between a number of (or perhaps most) commercial INR methods is poor and clinically too much INR variation was present, thus compromising the good care of the patients (9,10).
Hemker and colleagues (1,2) were the first to characterise the role of protein induced by vitamin K absence or antagonists (Pivka). They discovered that coumarin therapy is associated with an endogenous competitive coagulation inhibitor, which they later named Pivka. The proteins in question were pre-stages of vitamin K-dependent factors. They inhibited the prothrombin-converting complex, presumably against coagulation factor X (FX) (3). It was found that Pivka factors lack gamma carboxyglutamic acid, which is necessary for calcium binding and thereby for “adsorption” of these factors to phospholipid surfaces. Thus, they are inactive analogs to active coagulation factors (4,5).
Talstad contemplates the question why the standardisation of PT is a problem and suggests PT standardization by correction of the Pivka inhibitor (15,16). Heckemann et al. discloses a method for simultaneous determination of functional coagulation factors and competitive Pivka-inhibitors based on enzyme kinetics (17). Moreover, immunochemical assays for Pivka are also known (see, e.g., U.S. Pat. No. 5,516,640). However, no simple and practical method for Pivka correction in PT tests has been introduced in the prior art yet. Consequently, the aim of the present invention was to study the Pivka effect further and compare the Quick and the Owren PT methods in view of Pivka by using different reagents. As a result, the present invention provides a straightforward method for PT testing which overcomes the problem of the Pivka effect.